The solar cell has a voltage (V)−power (P) characteristics wherein power tends to increase as the amount of insolation increases as shown in FIG. 7 when the incident amount of insolation is taken as a parameter, provided that in FIG. 7 Pa, Pb, and Pc represent the maximum power points in the respective amount of insolation.
As a controlling method for extracting efficiently the maximum power through a photovoltaic inverter from the solar cell having such a characteristic, the so-called maximum power point tracking control method (MPPT control method) is used, wherein the photovoltaic inverter is controlled by generating minute changes in the load of this solar cell and some other means to monitor the ratio of the respective variation dP and dV (Δ, Δ=dP/dV) of the output power (P) and voltage (V) of the solar cell, and calculating the point where its polarity changes from positive to negative (or from negative to positive) (the maximum power point) so that the operating point of the solar cell may follow this point.
FIG. 8 is a circuit diagram showing an example of a conventional control device of a photovoltaic inverter based on the MPPT control method for controlling the output of solar cells. In this figure, 1 represents a solar cell, 2 represents a photovoltaic inverter for controlling the output of this solar cell, 3 represents an AC motor driven by the photovoltaic inverter 2, and 4 represents a mechanical load consisting of, for example, a pump, a fan or the like mechanically connected with the driving shaft of the motor 3 and rotatively driven by the same. The description below relates to the case of using a pump for the mechanical load 4.
This photovoltaic inverter 2 includes a voltage detector 21 for detecting the output voltage (V) of the solar cell 1, a diode 22 for blocking the countercurrent flowing from the inverter main circuit 25 described below, a current detector 23 for detecting the current (I) supplied from the solar cells 1, a capacitor 24 for smoothing DC voltage, an inverter main circuit 25 constituted, for example, by making a three-phase bridge connection of switching circuits constituted by connecting back to back a transistor and a diode, and a controlling device 26 for controlling to desired values of the voltage and frequency of the AC output of the inverter main circuit 25 based on the output voltage V and the output current I from the solar cell 1 detected by the voltage detector 21 and the current detector 23.
The control device 26 includes a maximum power point tracking (MPPT) monitoring circuit 31, a rotational speed instructing device 32, an adjustable speed controller 33, a function generator 34, and an inverter controlling circuit 35. The operation of this control device 26 will be described below with reference to FIG. 9.
In the first place, a start-up frequency instruction value fs is outputted from the adjustable speed controller 33 based on a start-up instruction to the solar cell 1 having a voltage−power (V−P) characteristic similar to the one shown in FIG. 9 depending on a given amount of insolation. Then, the inverter control circuit 35 performs pulse width modulation (PWM) operations based on voltage instruction values Vs corresponding to this start-up frequency instruction value fs and those corresponding to the frequency instruction value fs given by a function generator 34 that converts frequency instructions into voltage instructions by, for example, a function that converts the voltage/frequency ratio into a constant value. The control signals based on the result of such operations allow the ON/OFF control of the respective transistor constituting the inverter main circuit 25. Then, the inverter main circuit 25 generates AC power of a frequency and a voltage corresponding to the start-up frequency instruction value and the voltage instruction value and accordingly the motor 3 and the pump 4 start.
At this time, based on the values detected (V, I) respectively by the voltage detector 21 and the current detector 23 at previously fixed intervals, the MPPT monitoring circuit 31 calculates the variation (dP) of power (P, P=V×I) and the variation (dV) of voltage (V) and monitors the ratioΔ of the power variation to the voltage variation (Δ=dP/dV). However, when the motor 3 and the pump 4 starts as described above, the voltage of the solar cells 1 drops from a voltage V1 in the unloaded condition to a voltage in the loaded condition, and the Δ becomes negative (Δ<0). This is outputted in the rotational speed instructing device 32.
And the rotational speed instructing device 32 outputs a speed instruction value Δn (constant value) representing a speed increase or a speed reduction corresponding to the polarity of Δ mentioned above. In other words, when the Δ is negative (see FIG. 9), a speed increase instruction value in the form of a positive +Δn is outputted, and when the Δ is positive (see FIG. 9), a speed reduction instruction value in the form of a negative −Δn is outputted. Therefore, when the Δ is almost nil, the outputted Δn will also be almost nil.
And the adjustable speed controller 33 integrates Δn inputted by the rotational speed instructing device 32, and the calculation result is added to the start-up frequency instruction value fs to be outputted as a frequency instruction value. When the integrating time is adjusted by the polarity of Δn, the motor current is previously set at a given value so that the instructed speed may be attained as soon as possible without causing the motor current to turn into an overcurrent.
In other words, the voltage amplitude and frequency of the AC power outputted from the inverter main circuit 25 rises up to the voltage V2 which will be the maximum power point of the solar cells 1 shown in FIG. 9 in response to the output of the MPPT monitoring circuit 31 constituting a controlling device 26 after the motor 3 and the pump 4 started running by the AC power of the startup voltage and frequency from the inverter main circuit 25, and in response thereto the rotational speed of the motor 3 and the pump 4 increases.
When, for some reason, the voltage of the solar cells 1 dropped below the V2 mentioned above, in other words, when it is in a state of having passed the maximum power point, both the power P and the voltage V drop and therefore the ratio Δ of variation of the power and voltage outputted from the MPPT monitoring circuit 31 becomes positive (Δ>0) and a speed reduction instruction is given to the inverter. As a result, the motor 3 reduces its speed and consequently its power consumption, and the operating point shifts to the side of the maximum power point of the solar cell. Such a control of the photovoltaic inverter enables to make the loaded condition of the motor follow the maximum power point of the solar cell.
According to the conventional controlling method of photovoltaic inverter described above, due to the fact that the integrating time of + and − polarity in the adjustable speed controller 33 is set at a constant value with reference to a Δn of a constant value outputted by the rotational speed instructing device 32 in order to perform the MPPT control, the acceleration time (time required to accelerate unit revolutions) and the deceleration time (time required to decelerate unit revolutions) of the motor 3 in the vicinity of the maximum power point are relatively short, they tend to roam around the maximum power point and caused the control operation to fluctuate. And when the respective integrating time of the + and − polarities is set at a longer value in order to reduce this fluctuation, a new problem arises in that the settling time for the MPPT control and the response time accompanying a rapid change in the amount of insolation grow longer.
The object of the present invention is to provide a new method of controlling photovoltaic inverters that solves the above-mentioned problems.